786 TEE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



Hssure) passes back to disappear in the posterior extremity of the hemisphere (Fig. 

 423, 5). The fissure of Rolando separates the frontal from the parietal lobe, and 

 the portion of the brain comprised between its bifm^cation and the posterior 

 branch of the fissure of Sylvius, forms a quadrilateral lobe (Figs. 423, 13 ; 433, 

 10), which resembles the lobule of the curved plait in the brain of Man. 



Lastly, on the internal face of the hemispheres is found the inner portion of 

 the frontal and parieto-occipital lobes, and a long convolution that accompanies 

 the coi-pus callosum (Fig. 429), named the callosal convolution, or crested con- 

 volution {gyrus fornicatus), because of the notches on its upper border in Man. 

 It is separated from the frontal and parieto-occipital lobes by a deep groove 

 — the calloso-marginal fissure. 



The crested convolution commences, in front, beneath the genu of the corpus 

 callosum ; behind, it is inflected downwards, is continuous with the hippocampal 

 convolution, and by some annectent gyri is in relation with the posterior 

 extremity of the hemisphere. 



In fine, a cursory examination of the surface of the hemispheres denotes the 

 presence of some principal grooves limiting three lobes and a lobule ; these are the 

 frontal, parieto-occipital, and sphenoidcd lobes, and the lobule of the curved plait.^ 



We will now describe the convolutions of these lobes and lobule. 



b. Fronted lobe. — This lobe presents three faces. 



The inferior face (orbited lobe) is triangular (Fig. 424), and its base is occupied 

 by the fissure of Sylvius, in front of which is remarked the extra-ventricular nucleus 

 of the corpus striatmn (Fig. 424, 12), which has a portion of its surface perfora.'^ed 

 by vascular openings — the locus perforatus. Near the summit it detaches the 

 olfactory or ethmoidal lobide (Fig. 424, 15), which arises by two white-coloured roots 

 that margin the extra- ventricular nucleus ; the externcd root (Fig. 424, 13) is con- 

 tinuous with the convolution that is prolonged on the temporal lobe ; the internal 

 root (Fig. 424, 14), which is shorter, arises from the inner face of the hemisphere, 

 in front of the optic commissure. This appendicular lobule is directed forwards, 

 and terminates by an oval expansion — the optic bulb (Fig. 424, 16) — which 

 extends beyond the anterior extremity of the brain to be lodged in the ethmoid 

 fossa. The olfactory lobule has a cavity in its interior — a diverticulum of the 

 lateral ventricle — and is received into a depression of the frontal lobe, named 

 the olfactory fissure, that extends to the summit of the orbital lobule, and 

 separates two convolutions, the internal of which is named the gyrus rectus (Fig. 

 424, 21). 



The external face of the frontal lobe shows a great fissure almost parallel with 

 that of Rolando, and between these two fissures is a long convolution that usually 

 describes three curves (Figs. 423, 6, 6 ; 433, 11, 11) ; this limited convolution 

 takes the place of the ascending frontal convolution in Man. In front, it is 

 always united to the other frontal convolutions, and the fusion is more or less 

 apparent. 



On the same face is perceived another great fissure, that commences in the 

 vicinity of the crucial fissure, where it sometimes appears to be continuous with 



' In the domestic animals, the occipital lobe — already so diflBcult to circumscribe in Man — 

 Ib not more distinct in his neighbour. The posterior extremity of the hemispheres is, as it we-re, 

 pushed forward by the internal occipital prutubcranee and the cerebellum. We make of the 

 postt-rior region of the liemisphere a parieto-occipital lobe, implying by this term the fusion of 

 the two parietal and occipital lobes. The latter is certainly very small, but we cannot admit 

 its disappearance. We do not find in the brain of the Horse, the lobule of the insula or island 

 of Heil, which in Man is concealed at the bottom of the fissure of Sylvius. 



